1. Field of the Invention
The present invention relates generally to video signal recording and/or reproducing apparatus and more particularly is directed to a video tape recorder (CTR) of the helical scan type in which a magnetic tape is scanned by rotary magnetic heads along successive slant tracks.
2. Description of the Prior Art
In a standard video tape recorder (VTR), a pair of rotary magnetic heads having opposite azimuth gap angles are mounted on a rotary drum with an angular distance of 180.degree. therebetween, and a magnetic tape is wrapped around the rotary drum over a tape wrapping angle of substantially 180.degree.. In the recording made of such standard VTR, as shown in FIG. 1, recording tracks TA and TB are alternately formed in close side-by-side relation on a magnetic tape 1 by the pair of rotary magnetic heads HA and HB having different azimuth gap angles and such tracks extend obliquely or slant across the magnetic tape 1. A recording track conforming to the standard format can be formed by recording one field of a video signal in each recording track.
In order to reduce the diameter of the rotary drum of the standard VTR for permitting miniaturizing of the overall arrangement of such VTR, it has been previously proposed, as shown in FIG. 2, to mount four rotary magnetic heads, HD1, HD2, HD3 and HD4 on a rotary drum 2 with an equal angular distance of 90.degree. between the heads, and with the diameter of the rotary drum 2 being reduced to 2/3 the diameter of the rotary drum of the standard VTR. In such case, the magnetic tape 1 is wrapped around the rotary drum 2 with a tape wrapping angle of substantially 270.degree., as shown in full lines, and the rotational speed of the rotary drum 2 is increased to 11/2 times the speed used in the case of the two-head VTR.
If the azimuth gap angles of the first and third rotary magnetic heads HD1 and HD3 are selected to be a positive angle (for example, +7.degree.) and the azimuth gap angles of the second and fourth rotary magnetic heads HD2 and HD4, which are located at angular distances of 90.degree. from the first and third rotary magnetic heads HD1 and HD3, are selected to be a negative angle (for example, -7.degree.), each time the rotary drum 2 rotates three times, the rotary magnetic heads HD1, HD2, HD3 and HD4 sequentially scan four recording tracks TA, TB, TA and TB to thereby form a track pattern having the same format as is shown on FIG. 1.
This will be described more fully with reference to FIG. 3 in which a recording track TA1 is formed by the first rotary magnetic head HD1 having the azimuth gap angle of the HA head; a recording track TB2 is formed by the second rotary magnetic head HD2 having the azimuth gap angle of the HB head; a recording track TA3 is formed by the third rotary magnetic head HD3 having the azimuth gap angle of the HA head; and a recording track TB4 is formed by the fourth rotary magnetic head HD4 having the azimuth gap angle of the HB head. These four recording tracks TA1, TB2, TA3 and TB4, or four recording tracks corresponding thereto, are scanned by the magnetic heads HD1, HD2, HD3 and HD4 once each time the rotary drum 2 is rotated three times.
When the recording pattern of FIG. 3 with the same format as the standard recording pattern of FIG. 1 is formed on the tape 1 by the four rotary magnetic heads HD1, HD2, HD3 and HD4 mounted on the rotary drum 2 of reduced diameter, it will be apparent that the azimuth gap angles of the first and third rotary magnetic heads HD1 and HD3 are the same and the azimuth gap angles of the second and fourth rotary magnetic heads HD2 and HD4 are the same. For that reason, if the rotary magnetic heads HD1, HD2, HD3 and HD4 are made to track the recording tracks, in the reproducing mode, by the so-called control pulse (CTL) tracking servo system, there is no assurance that the recording tracks TA1, TB2, TA3 and TB4 recorded by the rotary magnetic heads HD1, HD2, HD3 and HD4 will be tracked by the same rotary magnetic heads HD1, HD2, HD3 and HD4 during reproducing. In other words, the tracking servo may lock onto a condition in which the rotary phase of the rotary drum 2 is displaced by an angular amount corresponding to two tracks, so that, for example, the recording tracks TA1, TB2, TA3 and TB4 are tracked by the magnetic heads HD3, HD4, HD1 and HD2, respectively, in that order.
In the above-mentioned CTL tracking servo system, a CTL signal (control signal) of logic level "1" or "0" is recorded in a track along a longitudinal edge of the magnetic tape and changes its logic level at each track in accordance with the recording azimuth gap angle of the respective one of the slant recording tracks TA1, TB2, TA3 and TB4. In the reproducing mode, such CTL signal is reproduced and used as the basis for tracking-control so that the recording tracks are reproduced by magnetic heads having the same azimuth gap angles.
Further, in a so-called 8 mm VTR now available on the market, the above mentioned control pulse (CTL pulse) is not recorded in a longitudinal track along an edge of the tape so that it becomes impossible to select the reproducing heads on the basis of a reproduced CTL pulse.
If a recording track is reproduced by a magnetic head different from the magnetic head used to record such track, variations in the angles between the rotary magnetic heads HD1, HD2, HD3 and HD4 mounted on the rotary drum 2 may cause so-called head mounting angle interference in the reproduced picture, that is, may cause reproduction of a skewed picture.
In this connection, if head mounting angles .theta..sub.12, .theta..sub.23, .theta..sub.34, and .theta..sub.41 (FIG. 2) between the rotary magnetic heads HD1, HD2; HD2, HD3; HD3, HD4; and HD4, HD1, respectively, contain head mounting angular errors .alpha.1, .alpha.2, .alpha.3 and .alpha.4, the head mounting angles .theta..sub.12, .theta..sub.23, .theta..sub.34 and .theta..sub.41 may be respectively expressed by the following Eqs. (1) to (4). EQU .theta..sub.12 =90.degree.+.alpha.1 . . . (1) EQU .theta..sub.23 =90.degree.+.alpha.2 . . . (2) EQU .theta..sub.34 =90.degree.+.alpha.3 . . . (3) EQU .theta..sub.41 =90.degree.+.alpha.4 . . . (4)
When the recording tracks TA1, TB2, TA3 and TB4 recorded by the rotary magnetic heads HD1, HD2, HD3 and HD4 mounted on the rotary drum 2 with such mounting angle errors .alpha.1 to .alpha.4 are respectively reproduced by the same rotary magnetic heads, the periods of the horizontal synchronizing signals contained in the reproduced signals obtained by switching from one to another of the heads HD1, HD2, HD3 and HD4, in turn, are coincident with those of the horizontal synchronizing signals inserted during recording. When each recording track is reproduced by the same recording head as was used to form that recording track, the apparatus is said to be operating in a head self-recording and/or reproducing mode, and the so-called head mounting angle interference or skewing will not appear in the reproduced picture.
However, if the recording tracks are reproduced by rotary magnetic heads different from the recording heads used to form the respective recording tracks, when the magnetic reproducing heads are switched, the periods of the horizontal synchronizing signals contained in the reproduced signals are displaced by amounts corresponding to the respective ones of the head mounting angle errors .alpha.1 to .alpha.4. As a result, as shown in FIG. 4, when the reproduced picture or image includes a straight line 5A to be displayed, for example, extending in the vertical direction of a picture screen 4, the occurrence of so-called head mounting angle interference causes a skewed picture in which the portions of the reproduced picture at opposite sides of the straight line 5A are expanded laterally, as shown by a dashed line 5B1 or 5B2.
In the reproducing circuit of the known VTR, the horizontal synchronizing signal contained in the reproduced signal is AFC (automatic frequency control)-controlled to thereby attempt to match its period with that of a predetermined reference synchronizing signal. However, in the reproducing mode, each time the rotary magnetic heads are switched, the period of the reproduced horizontal synchronizing signal may be never-the-less displaced for the lock-in time of the AFC loop. In other words, as shown on FIG. 4, the AFC control operation is not so effective at the upper portion of the picture screen 4, so that the skewing of the picture remains at such upper portion.
Further, when the recording track formed by a rotary head is not reproduced by the same rotary magnetic head, in addition to the above-described skewing of the picture, a so-called flicker may appear on the reproduced picture, for example, due to uneven rotation in each revolution of a motor, uneven amplitude of head output and scattering or variations of the frequency characteristics of the several heads.